Mobile devices, such as portable music players, gaming devices, and mobile computers or telephones, among other devices, are configured to generate, play back, or otherwise provide an audio signal using a speaker or an audio signal output port. A user-operated audio volume control can be provided to adjust an amplitude of the audio signal reproduced by the speaker or provided at the audio signal output port. The audio volume control can be integrated with the mobile source device and configured for manual operation by a user. The user-operated audio volume control can include a knob, button, slider, touchscreen feature, or other physical interface that can receive an input from a user and, in response, provide a control signal to an audio amplifier circuit or audio attenuator circuit to change the audio signal amplitude.
An audio signal can be processed in other ways to enhance a user's listening experience. For example, a mobile device can include a “loudness” feature that can adjust one or more frequency components of an audio signal, such as to subjectively improve a user's perception of the frequency content of the signal. A mobile device can include other frequency filters, such as to enhance intelligibility of an audio signal, such as by adjusting signal frequency components that correspond to the human voice range.
Another technique for enhancing a user's listening experience can include active noise reduction or noise cancelling. An active noise-cancelling device, such as a headphone device, can receive an audio signal of interest from an audio source device and can receive information about an ambient noise signal. The active noise-cancelling device can include circuitry that can cancel or attenuate a low frequency component of the ambient noise signal that is not part of the audio signal of interest. The effectiveness of a noise-cancelling device can be limited, and can depend on factors that are external to the audio source device, such as the bandwidth or frequency content of the ambient noise signal.